Cylinder blocks of two-cycle engines are normally fabricated by die casting. Recently there has been increased activity in casting the cylinder block by a lost foam process, in which the pattern is made from an evaporable polymeric material, such as polystyrene. In the typical lost foam process, the pattern is placed in a mold and an unbonded flowable material, such as sand, surrounds the pattern, as well as filling the cavities in the pattern. During casting, the heat of the molten metal will vaporize the pattern and the vapor will be entrapped within the interstices of the sand, while the metal fills the void created by evaporation of the foam material to provide a cast part identical in configuration to the pattern.
The casting of a two-cycle engine block has proven difficult due to the complexity of the porting and particularly the transfer passages. A typical two-cycle engine includes one or more transfer passages which provide communication between the crankcase and the upper ends of the cylinders. With a loop charge system, a combustible charge is drawn upwardly from the crankcase through a group of three transfer passages to the head end of the cylinder. The charges converge below the spark plug and the confluent charge is compressed by the upward stroke of the piston. After ignition by the spark, waste gases of combustion are expelled through the cylinder exhaust port with the downward power stroke of the piston. To enable the fuel-air stream to flow in the desired course to the head and then flow to the exhaust port to effect removal of the spent gases, the discharge end of the transfer passage must be positioned in precise tolerance with respect to the cylinder head.
It has been proposed to fabricate an evaporable foam pattern for a two-cycle engine block from a pair of sections, i.e. a crankcase section and a head section, which are joined along a parting line extending transversely through the axes of the cylinders. In this proposal, the end of the portion of each cylinder in the head pattern section adjacent the parting line is enlarged in diameter and the crankcase section is provided with a ring-like projection that extends longitudinally from the cylinder in the crankcase section and is received within the enlarged end portion in the head section. The ring-like extension is provided with a series of ears or lugs which engage ledges bordering the enlarged cylinder portion in the head pattern section. The undercut edges or recesses between the ears serve to define the lower edges of the transfer ports. The contiguous surfaces of the two pattern sections are joined together by an adhesive.
However, it has been found difficult to properly apply the adhesive to the contiguous surfaces due to the lack of visibility when the two sections are assembled. If the layer of adhesive is inadequate, the molten metal during casting will fill the void where adhesive is lacking, while on the other hand, if excessive adhesive is applied, the adhesive can extrude from the abutting surfaces into the transfer passages and the extruded glued joint will result in a bead of metal in the cast block, which can adversely effect the flow of the charge through the transfer passages.